Posted
by
Soulskill
on Monday April 19, 2010 @04:00PM
from the you-cannae-fire-while-cloaked dept.

KentuckyFC writes "Carpet cloaks took the world by storm last year because they were the first devices to hide objects at optical frequencies. The idea is that a thin layer of dielectric material placed on a surface can make light look as if it is reflecting off the original surface. In other words, the layer is invisible and anything embedded within it is invisible too. This trick is like hiding something under a carpet, hence the name. Carpet cloaks are relatively easy to make because the dielectric material does not need to be specially constructed to steer light in special ways; physicists call this an isotropic material. Now a group at MIT has shown that isotropic carpet cloaks have a fatal flaw. When viewed at an angle, the carpets don't hide objects at all. Instead, they simply shift their position by about the same distance as they are high. So when viewed from an angle of 45 degrees, an object 0.2 units high is shifted to one side by a distance of 0.15 units, says the team. That's a serious limitation for carpet cloaks."

This is only about 'carpet cloaks', not invisibility cloaks in general. The problem is that a carpet cloak is the optical analogue of simply putting a display screen in front of the object and a video camera behind the object. In other words, of course it doesn't bloody work from the side, you morons. A general invisibility cloak is still possible, but may require phased array optics or other exotic active techniques.

yeah, as long as we randomly modulate the shield frequencies, reverse the polarity of the heisenberg compensators, and amplify the transporter buffers... we should be good to go. Earl Grey tea never tasted so good.

yeah, as long as we randomly modulate the shield frequencies, reverse the polarity of the heisenberg compensators, and amplify the transporter buffers... we should be good to go. Earl Grey tea never tasted so good.

Now see here... If the polarity of anything is to be reversed, then clearly we should start with the neutron flow...

Well, cheer up. It might still mean that the Romulan's weapons hit some nearby console when they think they're targeting the warp core. Of course, it would be better if they didn't hit anything at all, but I'm affraid that the law that for each hit a console must explode in a shower of sparks and send some ensign flying across the room is more immutable than the laws of refraction;)

They don't have to be perfect; they just have to be good enough. Nor do that have to work all the time; they just need to work when needed, and for just long enough to allow the first shot. ("Spock, what's tha...doh!")

You probably meant "No, it's not. Not if the warhead is nuclear." Read the post. I said "almost". This cloak would be nearly completely effective against bullets. Nukes would not be.Man I get tired of you children.

You mean in real life, as opposed to internet dick waving arguments? That depends.

Depends on the size and type of warhead, the geology of the ground, the angle of the impact of the missile and the alignment of the tank relative to the strike.

A "standard" 1000lbs conventional cruise missile warhead is a fragmentation/blast effect weapon. Against a 60-ton MBT, anything more than a couple meters away will have minimal effects on the crew, assuming they're buttoned up. Shaken and dinged up, maybe.

OTOH, the tank itself may have been damaged and/or lost mobility but it's a far cry from having what's essentially a 1000 lbs land directly on the vehicle, in which case the tank is most assuredly dead and the crew does not survive.

In terms of the very basic science, what about the explosion is the dangerous bit? Fragmentation and the pressure wave, plus possible secondary fragmentation if you're inside a vehicle. Tank armor is designed to protect against these threats, which is why hardware designed to kill tanks are specialized to either penetrate armor or strike where there is next to no armor (the top).

Directly ON the tank, game's over. Next to the tank, now you're playing against the tank's strength, which is why I responded to badboy_2002 and interval1066 the way I did. A tank sittng right next to where the cruise missile hit is decidedly NOT about the same thing as a direct hit.

No, it's really not. Anti-tank weapons usually require a direct hit for a kill, or a very near hit to do significant disabling damage.

For example, - smaller anti-tank missiles like the US Javelin or Soviet RPG use shaped charges (HEAT) that need to have virtually direct hits for the superheated metal core to penetrate armor.
- air to surface missiles like the AGM-65 Maverick use kinetic energy to penetrate the target before exploding.
- the Hellfire and various other popular TOW system

No, it's about the size of the missile. The warhead is always listed in pounds but the potential is always listed in kinetic energy.

For example, the BrahMos has 32 times the kinetic energy of the Tomahawk, despite having a warhead 3/5 the size. It is by far much more destructive than our Tomahawk because of its higher mass and higher velocity capability.

Not if it realizes you're likely behind a cloak and just adjusts itself with some nifty math.

But... that was my first thought as well.

Okay, so I'm not invisible, but you still don't actually know where I'm at so its close enough for a lot of neat things.

I suspect however, that much like in the fantasy of StarTrek (sorry to burst some of your bubbles:) and root kit detection, theres always a way to detect the target, but knowing the right way to look for it is half the battle.

I assumed this applied to most of the Electromagnetic spectrum, or is it just visible light, do they say? I always naturally assume IR included in these kinds of things, since it is so close to visible light on the spectrum.

Radar on the other hand, I thought we had reliable countermeasures for, like jamming? or have I been watching too much Hollywood

They do look down at significant angles. This is why you have to hide stuff you don't want seen whenever the spy satellite is over the horizon, not just when its over head. The operators get to decide when to use fuel (or bleep off a flywheel) to reorient a satellite.

Actually, you would be wrong. the rules of engagement in Iraq were highly restrictive - and have been incredibly restrictive for the last several years - where you could not fire until fired upon, had clearly located and isolated the target, and cleared the counter attack with HQ. Otherwise you had to fall back. You can read more about the ROE for Iraq here [captainsjournal.com].

Well, they probably did and really knew about it for a long time, but geeks being geeks, these minor details probably didn't come out because they were so proud of what they had accomplished, and rightfully so. Even a cloak that works head on is freaking impressive to the point of becoming magic. I know they are just wave guides, but its still freaking impressive.

With that in mind, someone comes a long and notices it a long time later and points it out and the scientists are like 'yea well, we haven't got

So what if you took two of them and overlapped them at 45 degree angles? I realize that's not quite descriptive enough to clarify my line of thought, so let me elaborate:

Say I have a cube covered by this thing. When looking at it from a 45 degree angle, I see the cube displaced.

So what if I then take another slightly-larger carpet cloak and prop it up 45 degrees off-axis such that when I look at this outer cloak straight-on, I see "through" the outer cloak, and when I look at it at 45 degrees, I see

I mean, I know we all understand it, but if you're giving an example, why use unitless decimals when you can use integers and tangible concepts? Why not just say it would displace a 4 meter tall truck by 3 meters instead of 0.2 units tall object by 0.15 units?

1. Roll up the carpet.2. Put the object in a carpet, then put the carpeted object in another, slightly angled carpet, then put THAT into yet another slightly more angled carpet, and that entire batch into still yet another, even more slightly angled carpet, etc, until all angles are covered.

For all the talk of cloaking technologies I hear around here, this is the first I've heard of this one. Sure, I'm not an expert in the field, but if this "took the world by storm" last year, I'm surprised no news stories ever reached me.

Most suspicious, though, are the references to this as a technology for which practical devices have been built. The effect described in TFA is something you could see empirically if you had a working model; you don't need someone to draw a diagram showing the course of a

of circumstances where any viewpoint other than roughly straight on is impractical - for example looking at someone through a tunnel, or someone a long way off through a telescope where to get any viewpoint from a significantly different angle would take a lot of walking. Presumably it works just fine then.

The displacement isn't as useful as everyone thinks. Anything that is wider than it is tall will still get hit by any shot that's near center or to the correct side of the displacement, and some shots that would have missed will be hits. Actually anything whose height to width ratio is 4:3 or lower will still get hit. That covers a lot of the military equipment you'd want to hide - tanks, planes, ships, most buildings, most vehicles, anyone not standing up, etc.

Zhang and co go on to prove their assertion by tracing a ray that passes through the kind of isotropic carpet cloak that Pendry suggested. What they've discovered will shock carpet cloakers all over the world.